In general, two types of VTOL aircraft have been built and flown successfully: 1) rotary wing aircraft (helicopters) and 2) vectored-thrust, fixed-wing aircraft (e.g., the Harrier “Jump-Jet” and the JSF Joint Strike Fighter), in which the thrust generated by a turbojet engine is vectored downwardly for lift-off and then re-directed horizontally for wing-supported forward flight. Tilt-rotor vehicles such as the V-22 Osprey, in which vertically oriented rotors lift/lower the aircraft during takeoff/landings and rotate into horizontally oriented positions to propel the aircraft for wing-supported forward flight, are also known.
Although such aircraft serve their purposes with varying degrees of success, they are not without their limitations. For example, helicopters are limited in their forward speeds due to various aerodynamic considerations, including advancing/retreating blade aerodynamics. Additionally, the mechanical linkage systems by means of which power is transferred from the engines to the primary and tail rotors of helicopters are relatively complex and heavy. Similarly, the mechanical systems by means of which the engines of tilt-rotor aircraft pivot between their take-off/landing and forward flight orientations are extremely complex, and such tilt-rotor aircraft have not enjoyed widespread success.
Currently known fixed wing, vectored-thrust VTOL aircraft, on the other hand, are able to operate at much higher airspeeds than helicopters and tilt-rotor aircraft. Additionally, their thrust-vectoring systems are generally less complex and lighter than the pivoting systems found in tilt-rotor aircraft. Unfortunately, however, because current vectored-thrust VTOL aircraft lift and lower themselves by directing their engine exhaust directly downward, repeated operation of such aircraft in a given locale is limited to areas in which the ground surface has been specially prepared to withstand impingement of the hot gas exhaust. Furthermore, because such aircraft use the thrust of their engines directly and engine bleed-off air to respectively support and control the aircraft while in lifting/hover mode, which provides a relatively limited number of support posts or force points, control of such aircraft in hover mode can be relatively sensitive.
Further disadvantages of current VTOL systems may be summarized as follows. First, high power is required for VTOL in direct thrust systems. As a result, it is difficult to achieve sufficient margins for VTOL operation. Therefore, the engine is oversized, and significantly larger margins of engine thrust are required for VTOL than for efficient horizontal flight operation, and therefore range and efficiency of operation are reduced. Second, systems using direct engine thrust have high effluent velocities from engine jet (approximately 2400 feet/second) and high exhaust temperatures (approximately 1250° F.). As a result, direct thrust systems have problems with hot gas ingestion of re-circulating exhaust flow as it mixes with ambient air underneath the aircraft while in ground effect. This can cause sudden loss of thrust and vertical control as the hot gas can stall or surge the turbine engine. Third, systems using highly loaded lift fans (e.g. JSF shaft-driven lift fan) still have very high downwash velocity from the lift fan (approximately 800 feet/second). Fourth, current systems exhibit relatively poor control of VTOL attitude, since they typically use high-pressure bleed air from the engine, which reduces the engine thrust margin and delays control—particularly in roll and yaw (as in the use of thrust roll posts in the JSF). Fifth, there is generally poor, slow control of altitude and vertical velocity due to turbine engine spool-up and spool-down time, hence throttle lags. Sixth, there is a high potential for catastrophic failure, since there is no lift fan or engine redundancy in current VTOL lift or control systems.
In addition to the preceding, the following information may also be helpful to appreciating the current invention and the benefits and advantages of the invention.
Currently, the principal approaches for practical VTOL flight vehicles, some of which are referenced above, can be grouped into three broad categories: low- to medium-speed vehicles with rotors (helicopters, tilt-rotors, and similar aircraft); low-speed lifting fan or ducted fan vehicles with more than one lifting fan (such as a flying platform like the Paisecki Flying Jeep); and high-speed vehicles using jet thrust or variations of jet thrust and highly loaded lifting fans (e.g., the Harrier and the JSF). These approaches are differentiated by many factors, but one of the most significant factors is the thrust loading for the propulsion system that provides lift for takeoff and transition to wing-borne low-speed forward flight. The thrust loading (pounds per square foot loading of the propulsor), or power loading (pounds of thrust per shaft horsepower) of the propulsor(s), determines the power required to lift the aircraft and the velocity of the corresponding momentum stream (downwash) below it.
In many designs, the propulsion system is used to provide both vertical forces for hover and horizontal forces for forward flight. This can be done by tilting the VTOL propulsion device (e.g., the rotor in a helicopter or tilt-rotor) or by providing a separate propulsion device. Some implementations of VTOL aircraft such as flying platforms (e.g., the Paisecki Flying Jeep) generate horizontal propulsion forces by tilting the entire vehicle. Other implementations of VTOL aircraft (e.g., the UrbanAero X-Hawk and the vehicle illustrated in U.S. Pat. No. 6,464,166) use sets of vanes in the exit flow of vertically mounted fans. Developmental programs such as the Defense Advance Research Projects Agency's Canard Rotor/Wing project (Dragonfly X-50) seek to combine rotor technology for VTOL flight with high-speed flight, by stopping the rotor and propelling the vehicle in forward flight—supported on the stopped rotor—using conventional jet thrust to do so.